look out v.1. To take care; be careful; be on guard. ■ Usually used as a command or warning. "Look out!" John called, as the car came toward me."Look out for the train," the sign at the railroad crossing warns.2. To be alert or watchful; keep watching. A collector of antique cars asked Frank to look out for a 1906 gas head lamp.Compare: EYE OUT, ON THE WATCH. 3.informal To watch or keep (a person or thing) and do what is needed; provide protection and care. ■ Used with "for". Lillian looked out for her sister's children one afternoon a week.Uncle Fred looked out for his brother's orphan son until the boy was through college.Compare: LOOK AFTER.